1 15 ported or 2 15 sealed ???

i wanna run a 15 sx with a ed 9.1
when i was in my local shop the guy told me it would be better if i went with 2 15 sealed instead of one ported both getting the same rms 1 15 @ 1200rms ported and 2 15 @ 1200 rms sealed he said because of more cone area, and also will sound better and get louder. now i know he wasn't trying to sell me anything becasue he cant get re and i told him what i was getting. so what should i do. the ported box will be about 40x15.5x19 and the sealed would be the same (holding 2 of course)